1. Field of the Invention
The present invention relates to an ink jet recording method for recording on a recording medium, by discharging ink from discharge openings of a recording head, utilizing plural inks of a same color with different densities, and an apparatus therefor.
2. Related Background Art
In the conventional ink jet recording method, the record is formed by depositing ink droplets onto a recording material such as paper, by discharging ink from plural ink discharge openings formed on a recording head, based on data signals. Such recording method is utilized for example in a printer, a facsimile apparatus or a copying apparatus.
In such an apparatus there are known a method of utilizing an electric-thermal energy converting member in which a heat-generating element (electric-thermal energy converting member) is given an electric signal to locally heat the ink thereby inducing a variation in the pressure thereof and discharging the ink from the discharge opening, and a method utilizing an electric-mechanical converting member such as a piezoelectric element.
In such recording methods, a half tone recording is achieved by the dot density control in which the half tone is represented by the control of the number of recording dots per unit area, while the recording dots are maintained at a constant size, or by the dot diameter control in which the half tone is represented by the control of the size of the recording dots.
In general, the former dot density control is more popularly utilized, since the latter dot diameter control requires and is limited by the complex control method for delicately varying the recording dot size.
Also in case the ink discharge means consists of the electric-thermal energy converting member which can achieve a high resolving power because it enables easier manufacture and can be arranged with a high density, the above-mentioned dot density control is generally employed since the amount of variation in pressure is difficult to control, so that the diameter of the recording dot is difficult to modulate over a wide range.
One of the representative binarization methods employed in said dot density control for the continuous tone representation is the systematic dither method, but said method is associated with a drawback that the number of gradation levels is limited by the matrix size. More specifically, a larger number of gradation levels requires a larger matrix size, which leads to a larger pixel composed of said matrix in the recorded image, whereby the resolving power becomes deteriorated.
Another representative binarization method is the conditionally determined dither method, such as the error diffusion method, in which the threshold value is varied in consideration of the pixels surrounding the input pixel, in contrast to the above-mentioned systematic dither method, which is an independently determined dither method wherein the threshold value for binarization is determined independently from the input pixel. Said conditionally determined dither methods, represented by the error diffusion method, have certain advantages such as the compatibility of the gradation and the resolving power, and extremely little generation of the moire pattern in the recorded image in case the original image is a printed image, but is also associated with a drawback of granularity in the lighter area of the image, leading to a tendency that the image quality is evaluated as low. This drawback is particularly conspicuous in a recording apparatus with a low recording density.
For overcoming such granularity, there has been proposed a recording method, in an ink jet recording apparatus, of providing two recording heads for respectively discharging lighter-colored ink and darker-colored ink, and forming the recording dots with the light-colored ink from the low-density (light) level to the intermediate density level of the image, and with the dark-colored ink from the intermediate density level to the high-density (dark) level of the image.
FIG. 4 shows a principal part of a conventional color ink Jet recording apparatus of the serial printing type, employing such light and dark inks.
On a carriage 401 there are provided, with a predetermined mutual distance, a recording head Bkk for discharging dark black-colored ink, a head Bku for discharging light black-colored ink, a head Ck for discharging dark cyan-colored ink, a head Cu for discharging light cyan-colored ink, a head Mk for discharging dark magenta-colored ink, a head Mu for discharging light magenta-colored ink, a head Yk for discharging dark yellow-colored ink, and a head Yu for discharging light yellow-colored ink.
A recording material, consisting for example of paper of a thin plastic sheet, is supplied through transport rollers (not shown) and is pinched between discharge rollers 402, and is advanced in the illustrated direction by an unrepresented transport motor.
The carriage is guided and supported by a guide shaft 403 and an encoder. Said carriage reciprocates along said guide shaft, by means of a driving belt 404, driven by a carriage motor 405.
In the interior (liquid path) of each ink discharge opening of the above-mentioned recording heads, there is provided a heat-generating element (electric-thermal converting member) for generating the thermal energy for ink discharge.
An image can be formed by driving said heat-generating elements based on the recording signals, according to the timings read by said encoder, and discharging and depositing the ink droplets onto the recording material, in the order of dark black, light black, dark cyan, light cyan, dark magenta, light magenta, dark yellow and light yellow.
In a home position of the carriage, selected outside the recording area, there is provided a recovery unit having a capping portion 406, for maintaining stability of the ink discharge.
The above-explained light-dark multi-density recording method employing plural inks of different densities for a same color can improve the gradation particularly in the highlight area, and can reduce the granularity by the dots, thereby attaining higher image quality even by a simple increase of the density levels from two to three. This is because the noises felt by the single dots in the highlight area can be reduced by the deposition of lighter ink (with lower density) in such highlight area.
However, in the ink jet recording apparatus employing such light and dark inks, the stable ink discharge may become difficult, depending on the situation of use. If, among plural inks of different densities of a same color, at least an ink cannot be discharged in stable manner, the desired image can no longer be obtained.
In such ink jet recording apparatus utilizing such dark and light inks, the multi-value luminance signals of R, G and B colors supplied from a host equipment are converted in a color processing unit into multi-value density signals of Y, M, C and Bk colors, which are then divided, by a distribution table of a dot developing unit, into those to be recorded with the dark inks and those to be recorded with the light inks, and thus divided signals are binarized for recording by the recording heads. Consequently, if either of the dark and light inks is exhausted or becomes low in the remaining amount, the desired image can no longer be obtained even though the colors required for image formation are still present.
Also the stability of the ink discharge amount is significantly influenced by the temperature of ink or recording head and that of the atmosphere. In general the discharge amount is low when the ink temperature is low, and becomes larger as the ink is heated. As a result there has been encountered a drawback that the density of the recorded image varies depending on the temperature of the recording inks.